The present invention relates to a conductive polymeric composite material with a resistance which is self-regulated by the temperature. It is more particularly a fluoropolymer comprising a conductor, such as, for example, carbon black or any other electrically conductive material.
It is possible to render a composite conductive by incorporation of graphite in a polymer matrix. The application of a sufficient voltage results in heating by the Joule effect. In the absence of a circuit breaker mechanism, the temperature increases until the material is destroyed. The material of the present invention is based on a fluoropolymer comprising, as filler, a conductor, such as, for example, graphite, which shows an increase in the resistance as a function of the temperature (PTC or xe2x80x9cPositive Temperature Coefficientxe2x80x9d effect), so that the intensity stabilizes at an equilibrium temperature. This PTC effect thus makes possible thermal control of the intensity of the current. It exhibits numerous advantages by comparison with conventional resistances:
The control ofelectric heating systems is conventionally obtained by inclusion of a thermal circuit breaker in the circuit. In the event of failure of the circuit breaker, the circuit or the safety fuse is blown. The PTC material is self-controlled without it being necessary to include either a circuit breaker or a fuse.
The PTC heating system exhibits a reduced risk of combustion and of short circuit.
In the event of involuntary earthing of a PTC heating element region, short circuiting does not occur.
The PTC effect generates a moderate temperature, which is beneficial in 2 respects in comparison with conventional systems
The specifications imposed on insulating materials should be less strict;
The introduction of heat takes place over a more extensive surface area.
The composite material can be converted by the methods used in the plastics industry (coextrusion, moulding, and the like). It can also be applied as a paint to insulating substrates, whatever their geometry.
The prior art has disclosed two types of composite polymer systems which exhibit the PTC effect.
According to a first type, the PTC effect is based on the phenomenon of expansion of the polymer crystals disrupting the network of the conductive filler. The resistance of the composite slowly decreases when the amount of carbon black in a semi-crystalline polymer matrix is increased, to a concentration where the resistance falls. The latter represents a geometric transition which is known as the percolation threshold. It has been found that the maximum in the PTC effect corresponds to a critical concentration which is found in the vicinity of the percolation threshold. When the temperature of the material approaches the melting temperature of the matrix, an expansion in the crystalline region triggers the PTC effect. However, a high energy of the carbon black particles and a low shear modulus of the matrix result in a fall in resistance, known as the NTC (Negative Temperature Coefficient effect). This first type is described in the following references (CA denotes Chemical Abstracts):
131:243891 CA
TI Organic PTC thermistor materials with high transitive temperature
AU Yang, Fubiao; Li, Yongqin; Li, Xiaojun
CS Department 5, National University of Defense Technology, Changsha, 410073, Peop. Rep. China
SO Gongneng Cailiao (1998), 29(Suppl.), 724-725
CODEN: GOCAEA; ISSN: 1001-9731
PB Gongneng Cailiao Bianjibu
129:331461 CA
TI Effect of thermal treatment on crystallization and PTC properties of conductive PVDF/CB composite
AU Wang, Jikui; Wang, Gengchao; Zhang, Bingyu; Fang, Bin; Zhang, Zhiping
CS Inst. Mater. Sci. Eng., East China Univ. Sci. Technol., Shanghai, 200237, Peop. Rep. China
SO Gaofenzi Cailiao Kexue Yu Gongcheng (1998), 14(5), 93-95
CODEN: GCKGEI; ISSN: 1000-7555
PB xe2x80x9cGaofenzi Cailiao Kexue Yu Gongchengxe2x80x9d Bianjibu
125:277325 CA
TI Influences of crystallization histories on PTC/NTC effects of PVDF/CB composites
AU Zhang, Mingyin; Jia, Wentao; Chen, Xinfang
CS Dep. Materials Science, Jilin Univ., Changchun, 130023, Peop. Rep. China
SO J. Appl. Polym. Sci. (1996), 62(5), 743-747
CODEN: JAPNAB; ISSN: 0021-8995
104:121274 CA
TI Heaters
IN Shibata, Tsuneo; Nishida, Takeo; Terakado, Masayuki; Nitta, Isao
PA Matsushita Electric Industrial Co. Ltd., Japan
SO Jpn. Tokkyo Koho, 5 pp.
CODEN: JAXXAD
According to a second type, the PTC effect is based on the presence of two immiscible polymers. Among the materials of this type, PVDF/HDPE systems comprising carbon black as filler are the most well known. The PVDF and HDPE phases are immiscible and thus the PTC effect in this case depends very much on the morphology and on the distribution of the carbon black between these two phases. The carbon black is preferably dispersed in the HDPE phase, which becomes the conductive phase. If the PVDF is in good equilibrium with respect to the HDPE, the PVDF phase forms a specific structure which is favourable to the PTC effect. A remarkable distribution of the conductive PE phase in the PVDF phase is the condition for thwarting the NTC effect, which is produced at the melting point of the HDPE, which is lower than that of the PVDF. This second type is described in the following references:
131:287163 CA
TI Carbon black-filled immiscible blends of poly(vinylidene fluoride) and high density polyethylene: the relationship between morphology and positive and negative temperature coefficient effects
AU Feng, Jiyun; Chan, Chi-Ming
CS Department of Chemical Engineering Advanced Engineering Materials Facility, The Hong Kong University of Science and Technology, Kowloon, Hong Kong
SO Polym. Eng. Sci. (1999), 391(7), 1207-1215
CODEN: PYESAZ; ISSN: 0032-3888
PB Society of Plastics Engineers
130:96227 CA
Ti Carbon black-filled immiscible blends of poly(vinylidene fluoride) and high density polyethylene: electrical properties and morphology
AU Feng, Jeng; Chan, Chi-Ming
CS Dep. of Chemical Engineering, Advanced Engineering Materials Facility, The Hong Kong University of Science and Technology, Kowloon, Hong Kong
SO Polym. Eng. Sci. (1998), 38(10), 1649-1657
CODEN: PYESAZ; ISSN: 0032-3888
PB Society of Plastics Engineers
130:52964 CA
TI Carbon black-filled immiscible blend of poly(vinylidene fluoride) and high-density polyethylene: electrical properties and morphology
AU Feng, Jiyun; Chan, Chi-Ming
CS Department of Chemical Engineering, The Hong Kong University of Science and Technology, Kowloon, Hong Kong
SO Annu. Tech. Conf.-Soc. Plast. Eng. (1998), 56th(Vol. 2), 2476-2480
CODEN: ACPED4; ISSN: 0272-5223
PB Society of Plastics Engineers
and, finally, Patent Application WO 9805503.
It has now been found that a composite material composed (i) of a blend of PVDF homopolymer or copolymer crystallized essentially in the xcex2 form and (ii) of a conductive filler, such that the crystals in the xcex2 form are nucleated on the surface of the particles of the conductive filler, exhibits the PTC effect but according to a mechanism different from the prior art.
The PVDFs used in the composite crystallize in the xcex2 form (or the I form). This is a piezoelectric polar crystalline arrangement, the crystals of which are capable of being oriented in the direction of the electric field and contributing to the transportation of charges. For example, as regards a PVDF copolymer (VF2, HFP and TFE), this morphological arrangement is disrupted with the temperature as the surface of the crystals, rich in HFP and TFE units, is disturbed by the melting, the transfer of charges between particles of the conductive filler slows down and the resistance increases, which is reflected by the PTC effect.